The present invention relates to a nuclear fuel assembly for a fast breeder reactor, a reactor core wherein the nuclear fuel assembly is used as one of components, and a regulating method of coolant distribution of the reactor core, especially, to the core which is preferable to reduce a pressure drop of coolant of the reactor core of the fast breeder reactor and to improve thermal characteristics of the reactor core.
As illustrated in FIG. 3, a nuclear fuel assembly (hereinafter called a fuel assembly) for a fast breeder reactor of the prior art was generally of a type of a fuel assembly which was composed of a wrapper tube 2 wherein a plurality of nuclear fuel rods 1 were arranged in triangle lattices with wire spacers 5 or grid spacers between an upper shield 6 and a lower shield 7. The fuel rod was composed of a cladding tube 4 with a small diameter containing a stack of nuclear fuel pellets 3. The wrapper tube 2 has a uniform inner width in all through the axial direction (vertical direction) of the fuel assembly. The reactor core of the fast breeder reactor was composed of a plurality of the fuel assemblies which were standing together vertically in the core region.
The nuclear fuel pellets 3 can be divided into a blanket fuel pellet for breeding and a driver fuel pellet for driving the reactor. The blanket fuel pellets are inserted into each of an upper and a lower blanket region of the cladding tube 4, and the driving fuel pellets are inserted into a core region of the cladding tube 4. A gas plenum portion is formed at lower portion lower than the lower blanket portion of the cladding tube 4.
A fuel assembly having a wrapper tube free structure wherein a structural material other than a part of or all of the wrapper tube 2 is deleted is also known.
A reactor core is composed of a plurality of fuel assemblies which are standing together vertically in coolant. The fuel assembly comprises a plurality of nuclear fuel rods and a wrapper tube as a means of fuel containment.
As for a reactor other than a fast breeder reactor, a technique for a boiling water reactor was disclosed in the JP-A-1-98994 (1989) and the JP-A-59-180389 (1984). The technique described above is close to the present invention in a point of an altered inner width of a nuclear fuel rod container (which is called a channel box in the technical field of the boiling water reactor). In the technique disclosed in the JP-A-1-98994 (1989), an inner width of the channel box is enlarged at a down-stream side of coolant while an outer width is maintained uniform. In the technique disclosed in the JP-A-59-180389 (1984), both of an inner and outer width of a channel box is enlarged continuously from an up-stream side of the coolant to a down-stream side of coolant in a reverse tapered shape.
As general characteristics of a type of fuel assembly which had a wrapper tube 2 like a fuel assembly for a fast breeder reactor, the fuel assembly had to have a structure to wrap around a plurality of nuclear fuel rods 1 having a small diameter with a wrapper tube 2 and to settle a small gap between the fuel rods in order to make a reactor core compact and to improve a nuclear characteristics of the reactor, hence a large powered circulating pump was used for coolant circulation because of a large pressure drop of coolant of coolant flowing through a fuel bundle portion, wherein a plurality of nuclear fuel rods 1 are bundled, of the wrapper tube 2.
And the wrapper tube 2 had to have a thick wall to sustain the fuel assembly structurally and to depress expansion in horizontal direction caused by neutron radiation and pressure difference between the inner side and the outer side of the wrapper tube 2 in operation. Further, the wrapper tube had a structure to maintain a gap properly between each next wrapper tube 2 even though the fuel assembly expanded in horizontal direction as described above in composing the reactor core by standing a plurality of fuel assembly together vertically in the core region of the reactor.
One of the distinguished features of a fast breeder reactor from other type reactors is a phenomenon to cause a dislocation of chemical elements in materials of the wrapper tube and expansion at a portion of the wrapper tube (a middle portion of the wrapper tube) which is faced to a reactor fuel region by the neutron irradiation as burning of the nuclear fuel proceeds. Once the expansion is caused, as the wrapper tube has a closed shape in a horizontal section, both of the inner and the outer width of the wrapper tube are enlarged and a situation wherein a wrapper tube contacts with adjacent wrapper tubes is easily caused.
The phenomenon of expansion is called a swelling. The swelling is more distinguished in the fast breeder reactor than in other reactors, and the swelling is different from a channel creep caused by a stress which is generated by a pressure difference between an inner and an outer side of a wrapper tube in other type reactors. In the fast breeder reactor, the middle portion of the wrapper tube expands in a horizontal direction by both of the stress generated by the pressure difference and the swelling. Therefore, although the gap between each of wrapper tubes standing together vertically in the reactor core might be settled closer at regions near both ends of the wrapper tube, a large gap was settled actually in consideration of the expansion in horizontal direction at the middle portion of the wrapper tube in the fast breeder reactor. The gap between each of next wrapper tubes 2 had been maintained with a pad 8 having a larger width than the outer width of the wrapper tube 2 at the portions of the upper and the lower shield 6, 7, locations. The setting of the wrapper tube 2 and the gap between each of wrapper tubes 2 resulted in a reduction of a flow area in the fuel assembly, and had been a cause to increase pressure drop of coolant of the reactor core.
If pressure drop of coolant at the portion of fuel rod bundle in the fuel assembly can be reduced, a large reduction of pressure drop of coolant of reactor core as a whole becomes possible.
On the other hand, a fuel assembly of wrapper tube free type has less pressure drop of coolant as much as an amount of pressure drop of coolant caused by wrapper tubes, but leakage and unbalancing flow of coolant among fuel assemblies are concerned because wrapper tubes are not existing in the fuel assembly even though distribution of coolant to each fuel assemblies composing of the reactor core is regulated in corresponding to output power of the each fuel assemblies, and an investigation on the composition of the fuel assembly is necessary to resolve the concerning.
In a case wherein a technique disclosed in the JP-A-1-98994 (1989) is applied to a wrapper tube for a fast breeder reactor, a plurality of wrapper tubes have to be standing together vertically in a core region with the same large gap as a conventional reactor core in consideration of swelling etc. as the wrapper tube has a uniform outer width all through the total length from the top to the bottom without any exception at middle, upper and lower portion of the wrapper tube. Therefore, a coolant flow area in the wrapper tube has to be reduced reversely as much as a plurality of wrapper tubes occupies a wider area for standing in a limited core region with a large gap between each of wrapper tubes, and consequently, an increasing of pressure drop of coolant of the reactor core is easily induced.
In a case wherein a technique disclosed in the JP-A-59-180389 (1984) is applied to a wrapper tube for a fast breeder reactor, a lower end of the wrapper tube is reduced in width, and the reduced portion of the lower end of the wrapper tube is fixed to a member composing an inlet of coolant (equal to the lower shield in the fast breeder reactor), and the width of the reduced portion will not be enlarged even though burning of the fuel is proceeded. Therefore, the reduced portion in width is maintained all through the burning period of the nuclear fuel from the beginning to the end of the period, and will be easily a cause to increase the pressure drop of coolant of the reactor core.
Further, a technique to enlarge a width of a middle portion of a wrapper tube by placing a reverse tapered tube is equal to abandon a margin to accommodate expansion in the horizontal direction by swelling etc., which will be caused in the middle of burning period, from the beginning of the burning of the nuclear fuel as much as an amount of enlarged width, and a gap between each of the wrapper tubes has to be enlarged in order to avoid contact with adjacent fuel assemblies even though the expansion in the horizontal direction is occurred. Accordingly, a large area for the reactor core becomes necessary, and the technique is not suitable for the reactor core having a limited area.
Therefore, in the technical field of the fast breeder reactor, the development of a fuel assembly which enable to distribute flow of coolant exactly and to decrease pressure drop of coolant in a reactor core having a small area as possible has been desired.